1. Field of the Invention
The present invention relates to a Raman amplifier having a large gain bandwidth, and more specifically to a Raman amplifier having a gain bandwidth larger than a bandwidth corresponding to a Raman shift frequency.
2. Description of the Related Art
In a large optical communications system, a repeater including an optical amplifier for amplifying an optical signal is normally provided in a transmission line. In a large-capacity optical communications system, a WDM (wavelength division multiplexing) transmission is often used, and a Raman amplifier receives much attention as an optical amplifier for amplifying WDM light.
FIG. 1 shows a basic configuration of an example of a Raman amplifier (for example, SubOptic '2001 PD5, H. Nakamoto et al. “1.05 Tbit/s WDM Transmission over 8186 Km Using Distributed Raman Amplifier Repeaters”). In Raman amplification, a transmission medium (optical fiber) for transmission of a signal is used as an amplification medium. The Raman amplifier includes a pump light source 1 for generating pump light, and a WDM coupler 2 for guiding the pump light to optical fibers 3 and 4 for use as amplification media. The pump light source 1 is, for example, a laser light source. Also in the Raman amplification, normally, the smaller the core diameter of an optical fiber used as an amplification medium, the larger the Raman gain. Therefore, in the Raman amplifier shown in FIG. 1, a optical fiber (−D) 3 having a small core diameter and negative dispersion is provided.
In the Raman amplifier with the above-mentioned configuration, the signal (WDM light) transmitted through the optical fiber (+D) 4 passes through the optical fiber (−D) 3. At this time, pump light is supplied to the optical fiber (−D) 3. That is, the optical fiber (−D) 3 functions as an amplification medium. Therefore, the signal light is amplified in the optical fiber (−D) 3. The Raman amplifier is less noisy and obtains larger gain band than the most popular optical amplifier in the current market, that is, an erbium doped fiber amplifier (EDFA).
FIG. 2 shows the gain characteristic of the Raman amplification. This example shows the gain characteristic obtained when the pump light having a wavelength of 1 μm is supplied to a quartz optical fiber.
In the Raman amplification, when pump light is supplied to an optical fiber, the optical fiber functions as an amplification medium which amplifies the light of the frequency shifted from the frequency of the pump light by a predetermined frequency.
“Practically, as shown in FIG. 2, the Raman gain in the optical fiber grows substantially linearly with an increasing amount of frequency shift from the pump light frequency, and indicates the peak at the frequency about 13.2 THz lower than the pump light frequency (in the case of quartz fiber).”When the amount of frequency shift from the pump light frequency exceeds 13.2 THz, the Raman gain suddenly decreases. Hereinafter, the difference between the frequency of the given pump light and the frequency indicating the peak of the Raman gain obtained from the pump light is referred to as a Raman shift frequency. That is, the Raman shift frequency is approximately 13.2 THz.
FIG. 3 shows the amplifying operation of the Raman amplifier. In FIG. 3, it is assumed that the backward-pumping system in which pump light is propagated in the opposite direction of the propagation of signal light.
In FIG. 3, the transmission station or the repeater at the preceding stage outputs signal light of predetermined optical power (−8.2 dBm/ch in this example). In this case, the optical power of the signal light gradually attenuates as the propagation distance becomes longer. If there is no Raman gain, the optical power of the signal light which has reached a repeater 5 attenuates down to −18.5 dBm/ch. Therefore, if there is no Raman amplifier, it is necessary to obtain the gain of about 10 dB using an erbium doped fiber amplifier, etc. to set the output power of the repeater 5 at −8.2 dBm/ch. When only an erbium doped fiber amplifier is used, the optical signal power level in the transmission line drops to −18.5 dBm/ch. On the other hand, when the Raman amplifier is used in addition to the erbium doped fiber amplifier, it drops only to −13.7 dBm/ch. Thus, the noise characteristic can be improved by using the Raman amplifier.
FIG. 4 shows an example of the arrangement of wavelengths using the Raman amplifier. In this example, the Raman amplification is performed using plural variations of pump light having different frequencies. That is, the Raman amplifier uses plural variations of pump light having different wavelengths λ1˜λ4. In the Raman amplification, as described above by referring to FIG. 2, the peak of gain appears in the frequency area of about 13.2 THz shifted from the pump light frequency. In the 1,550 nm band, 13.2 THz can be converted into wavelength, which corresponds to approximately 100 nm. Therefore, in the Raman amplifier, the peak gain appears in the frequency area about 100 nm shifted from each pump light wavelength. When the gains by the plural variations of pump light λ1˜λ4 are compounded, a large gain band (about 100 nm in this example) can be obtained.
However, in the Raman amplification, as described above by referring to FIG. 2, a certain gain can be obtained in an area smaller than 13.2 THz in frequency shift from the pump light frequency. That is, in the Raman amplification, a gain can be obtained in an area smaller than 100 nm in wavelength shift from the pump light wavelength. Therefore, there can be a phenomenon in which pump light having a short wavelength amplifies pump light having a long wavelength. Hereinafter, the phenomenon can be referred to as pump-to-pump.
When a pump-to-pump phenomenon occurs, the optical power of pump light having a long wavelength (pump light to be pumped) becomes large, but the optical power of pump light having a short wavelength (pump light for pumping) decreases correspondingly. As a result, the noise characteristic improving effect by the Raman amplification of the signal light having a short wavelength (mainly the signal light amplified by the pump light having a short wavelength) is reduced as described below.
FIGS. 5A and 5B are explanatory views of the influence by a pump-to-pump phenomenon. FIG. 5A shows the state of the signal light power in which no pump-to-pump phenomenon occurs. The Raman amplifying operation in this case is described above by referring to FIG. 3. FIG. 5B shows the state of the signal light power in which a pump-to-pump phenomenon has occurred. If a pump-to-pump phenomenon has occurred, a part of the energy of the pump light having a short wavelength is used to amplify the pump light having a long wavelength. Thus, in the Raman amplifier in which a pump-to-pump phenomenon has occurred, it is necessary to increase the optical power of the pump light so that the signal light can be amplified as in the case in which no pump-to-pump phenomenon has occurred. Therefore, the noise from the ASE (amplified spontaneous emission) increases, thereby reducing the noise characteristic. When a pump-to-pump phenomenon occurs, the optical power level is quickly enhanced around the fiber length of 50 km. Therefore, the lower limit of the optical power level in the fiber drops, thereby reducing the above-mentioned noise characteristic improving effect.
If the optical power of each pump light is changed by the pump-to-pump phenomenon, the gain in the wavelength area for transmission of signal light does not become flat, and there is the problem that it is difficult to maintain uniform optical power of each signal light. Furthermore, since a part of the energy of the pump light having a short wavelength is absorbed by the pump light having a long wavelength, it is also necessary to keep sufficient optical power of the pump light having a short wavelength. Therefore, since it is necessary to wavelength-multiplex plural variations of pump light, the resultant configuration becomes exceedingly complicated.
In this situation, there is a method of suppressing a pump-to-pump phenomenon by appropriately modulating each pump light to solve the above-mentioned problem (for example, the Non-patent application document 1).
FIG. 6 shows a well-known pump light modulation method described in the non-patent application document 1. In FIG. 6, the horizontal axis indicates time while the vertical axis indicates an optical power level. In the Raman amplifier, pump light having a short wavelength (pump light 1 (1,423 nm) and pump light 2 (1,444 nm)), and pump light having a long wavelength (pump light 3 (1,464 nm) and pump light 4 (1,495 nm)) are used. Each pump light is modulated by the duty of 50 percents.
The pump light having a short wavelength and the pump light having a long wavelength are modulated to have opposite phases. That is, when the pump light having a short wavelength is in the emission state, the pump light having a long wavelength is in the extinct state. When the pump light having a short wavelength is in the extinct state, the pump light having a long wavelength is in the emission state. Therefore, if the wavelength dispersion in an optical transmission line is ignored, no pump-to-pump phenomenon occurs between the pump light having a short wavelength and the pump light having a long wavelength. On the other hand, the phases of the pump light belonging to the same group are the same. That is, the phases of the pump light 1 and 2 are the same, and the phases of the pump light 3 and 4 are the same. Therefore, there can be a pump-to-pump phenomenon occurring in the pump light belonging to the same group. However, the wavelength difference between the pump light belonging to the same group is approximately 20˜30 nm, and the transfer of energy in the pump light is small. Therefore, when the pump light modulation as shown in FIG. 6 is performed, a pump-to-pump phenomenon can be suppressed on the whole.
Non-patent Application Document:
OFC 2002, WB4 C. R. S. Fludger et al., “Novel Ultra-broadband High Performance Distributed Raman Amplifier Employing Pump Modulation” (FIG. 1)
Recently, a larger-capacity WDM signal is demanded, and an optical transmission system having a wavelength area larger than 100 nm for arrangement of signal light has been developed. In this system, as shown in FIG. 7, it is necessary to appropriately arrange plural variations of pump light over a wavelength area larger than the wavelength area corresponding to the Raman shift frequency.
However, the non-patent application document 1 is described based on the presumption that the wavelength area for arrangement of signal light is smaller than the wavelength area corresponding to the Raman shift frequency. Therefore, if the method of the non-patent application document 1 is applied to the Raman amplifier having a wavelength area for arrangement of signal light larger than the wavelength area corresponding to the Raman shift frequency, then the pump-to-pump phenomenon cannot be appropriately suppressed, and it is considered that a desired noise characteristic cannot be obtained. That is, when a wavelength area for arrangement of signal light is larger than the wavelength area corresponding to the Raman shift frequency, the well-known technology cannot sufficiently suppress the noise from the pump-to-pump phenomenon.